Mineralogical transformations in the Fe-laterite profiles of Saudi Arabia: A study of weathering dynamics and secondary lateritization

Abstract

This study examines an iron-rich laterite profile, 4 to 9 meters thick, located between the ancient Precambrian Arabian Shield rocks and the younger Phanerozoic layers in Saudi Arabia. The profile begins with hard parent rocks composed of quartz diorite and gabbro containing iron-silicate. As these rocks weather, they become progressively softer and more clay-like, forming slightly and highly weathered argillaceous rocks. The Fe-laterite profile includes three main layers: the original parent rocks, the slightly altered rocks (saprolite), and the highly altered argillaceous rocks at the top. Samples from different layers of the weathering profile were collected and analyzed for their mineral content. The original quartz diorite and gabbro primarily consist of feldspars, hornblende, chlorite, and quartz. In the slightly altered layer, the hornblende and chlorite expand and peel apart, while the feldspar crystals transform into kaolinite and sericite. In the highly altered top layer, the remaining iron-silicate minerals show significant curling and breaking apart, and the surrounding clay becomes more uniform, composed mainly of mixed dark iron-oxyhydroxides and a light aluminum- and silicon-rich gel with small amounts of quartz. The study explores the progressive stages of weathering, including: a) Initial breakdown of iron-silicate minerals through hydrolysis and oxidation, b) Further destruction of these minerals and the formation of goethite, hematite, and kaolinite, c) Final stages where kaolinite and small microcrystalline quartz aggregates form from silica leached from higher layers, a process known as secondary lateritization.